This invention relates to a method and an apparatus for sintering by which a sintered ceramic compact or a ceramic film having a compact structure (i.e., dense structure) can be produced.
Conventionally, a compact of particulate material for a ceramic, a compact of particles of metal, or a ceramic precursor film, is sintered by heating it at a high temperature in any one of various ambient gases (arbitrary ambient gases), wherein no additional pressure is applied to it. Further, to make the material dense and to enhance its mechanical, electrical, magnetic, and optical properties, a sintering technique has been developed wherein a mechanical force or a force via gas media is applied to the material to produce a sintered product made of fine crystal particles. For example, the hot pressing method and the sinter forging method are known. They each sinter a material while applying an axial mechanical force to it. In these methods a pressure is applied by a push rod to the material in its axial direction while it is being sintered.
However, in these methods the surface of the material that has contacted the push rod or die is contaminated. Thus, they require polishing or cutting off the surface of the material after it is sintered. Thus a high cost is necessitated to carry out the process.
The hot isostatic pressing (HIP) process is known as a method wherein a force is applied to a material via a gas media. Further, there are two methods in this process, namely, 1) the capsule HIP method, wherein a compact of the material is sealed in an evacuated capsule and then heated in a high-pressure gas under the HIP and 2) the capsule-free HIP method, wherein a material that has a density equal to or greater than 90% of the theoretical density is heated in a high-pressure gas.
The capsule HIP method has an advantage when producing a dense sintered product at a low temperature and a sintered product that has fine particles, due to the effect of the high-pressure gas. However, it has a problem in that it requires additional costs and time to put the material into the capsule and later remove it from the material. Further, although the capsule HIP method has another advantage, i.e., to make a large amount of material dense at once, it has a problem in that it must make the density of the material equal to or greater than 90% of the theoretical density by using an additional process, i.e., it requires a two-stage sintering process.
On the methods for producing a thin film, they are roughly classified into two methods, namely, the liquid phase method and the gas phase method, typically depending on the initial material. The representative liquid phase method is the sol-gel method. In this method an alkoxide liquid, etc., are mixed to be a predetermined composition, the resultant mixed solution is applied to a monocrystalline substrate such as Si, SrTiO3, etc., to coat it, and the coated monocrystalline substrate is heated in an electric furnace to form a ceramic film. The liquid phase method such as this sol-gel method, etc., has an advantage in that it can precisely control the chemical composition of a precursor film material at a stage before it is heated. However, since the liquid phase method requires a heating process at high temperatures to make the ceramic film dense, there has been a problem of occasionally causing a change in the chemical composition of the ceramic film due to the chemical species evaporation in the precursor film. Thus the development of a sintering method that can make a film such as a precursor film, etc., dense at lower temperatures has been desired.
Further, in addition to that problem, it has been reported that it causes defects of cracks, etc., in the film when heating it, depending on the composition of the material of the produced film and on the kind of substrate (the kind of material and the roughness of the surface of the substrate). These defects, which are caused in the film during heating, are considered to be caused by the great stresses caused in the surface (X-Y plane) of the substrate by the shrinkage of the volume of the material during heating. Therefore, the present research has tried to produce a dense (compact) film by controlling the rate of heating, etc. Although in this case the above-mentioned hot pressing method and HIP method can be applied to make the film dense, it cannot always be a suitable process to make a film dense since it involves some problems such as a pretreatment and removal of a surface phase of a material.
Accordingly, the development of a sintering method and apparatus that can easily produce, without having said problems, a sintered ceramic compact and a ceramic film, each of which have a compact (dense) structure, has been desired.
The inventors of this application have considered said problems and have recognized that it is an important object to develop a sintering method that can apply stresses to a material in a predetermined direction without touching it when sintering a compact of particulate material for a ceramic or of metal particles or a ceramic precursor film After a difficult review and research, the inventors have found that a dense sintered compact of a ceramic, a dense sintered metal compact, or a dense ceramic film, can be produced by heating and baking a work while applying a centrifugal force to it by rotating it at a high speed, and thus they have conceived the present invention.
The present invention, which has been conceived to resolve said problems, is a method for sintering a compact made of material of particles of ceramic or metal, or a ceramic precursor film by heating and baking it while applying a centrifugal force to it. The present invention also includes an apparatus for sintering, comprising a furnace; a work-holding portion rotatably mounted in the furnace for holding material to be sintered; and rotating means connected to the work-holding portion for rotating it so as to apply centrifugal force to the material held by it.
Accordingly, a centrifugal force is generated when the work is rotate data high speed in the heating device. This centrifugal force is applied to a material that is attached to the work-holding portion while heating and baking the material. Thus stresses are applied to the material held by the work-holding portion, thereby having the particles be at the most appropriate degree of compaction, while its sintering proceeds. Further, the apparatus of this invention may include a vacuum and magnetic shielding bearing that rotatably receives the work-holding portion. The use of the vacuum and magnetic shielding bearing allows a material to be sintered in any one of various ambient gases, such as in a vacuum or an arbitrary, pressurized ambient gas.